In its general aspect the present invention relates to a process for urea production.
The present invention relates specifically to a process for urea production of the type comprising the steps of:
performing a reaction between ammonia and carbon dioxide in a reaction space to obtain a reaction mixture comprising urea, carbamate and free ammonia in aqueous solution;
subjecting said mixture to a treatment of partial decomposition of the carbamate and partial separation of said free ammonia in aqueous solution to obtain a first flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising urea and residual carbamate in aqueous solution;
subjecting said first flow comprising ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a first portion of carbamate in aqueous solution;
recycling said first portion of carbamate to said reaction space;
feeding said flow comprising urea and residual carbamate in aqueous solution to a urea recovery section;
separating in said recovery section said residual carbamate from the urea to obtain a second portion of carbamate in aqueous solution.
The present invention also relates to a plant for carrying out the above mentioned process and to a method for modernizing an existing urea plant to obtain a plant according to the present invention.
As known, in the field of urea production the need is ever more growing of plants having greater capacity and operating flexibility on the one hand, on the other hand, requiring ever smaller investment and operating costs, in particular in energy terms.
To this end, there have been proposed and implemented in the art a series of urea production processes essentially based on the performance of a conversion reaction in a reaction space fed with ammonia (NH3)and carbon dioxide (CO2) and to which are recycled the unreacted substances contained in the urea solution leaving the reaction space, in particular ammonia, carbon dioxide and carbamate in aqueous solution.
A process of this type is shown in FIG. 1, and comprises downstream to a reaction space, a carbamate decomposition unit and a urea recovery section for separating from the urea solution the unreacted substances to be recycled.
If, on the one hand, this recycle allows almost complete recovery of valuable substances such as ammonia and carbon dioxide, on the other hand it also involves the sending to the reactor of large quantities of water (H2O) which are detrimental to the overall yield of conversion of the carbon dioxide to urea, with the yield being generally between 59% and 63%.
The technical problem underlying the present invention is accordingly to conceive and make available a process for urea production achieving high conversion yield which would be technically simple to implement and would involve low investment and operating costs.
In accordance with the present invention, this problem is solved by a process of the above mentioned type which is characterized in that it comprises the additional steps of:
subjecting at least part of said second portion of carbamate in aqueous solution obtained in said recovery section to a treatment of partial decomposition to obtain a second flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising residual carbamate in aqueous solution;
subjecting said second flow comprising ammonia and carbon dioxide in vapor phase to at least partial condensation to obtain a third portion of carbamate in aqueous solution;
recycling said third portion of carbamate to said reaction space.
According to this invention, at least part of the carbamate in aqueous solution leaving the urea recovery section is advantageously subjected to a treatment of partial decomposition separating unreacted ammonia and carbon dioxide from a solution rich in water comprising residual carbamate.
So doing, the unreacted substances which are recycled to the reaction space have a very low water content, and thus it is possible to substantially limit the water fed to the reaction space, permitting a high conversion yield.
In order to obtain a high degree of decomposition of said at least part of the second portion of carbamate in aqueous solution, the same is preferably subjected to a decomposition treatment at a pressure substantially corresponding to the pressure in the reaction space.
To improve and assist the condensation and separation steps of the unreacted substances in the urea recovery section, the flow comprising residual carbamate in aqueous solution resulting from the treatment of partial decomposition of the second portion of carbamate is advantageously fed to said urea recovery section.
According to another embodiment of the present invention, the process comprises the steps of:
feeding the reaction mixture comprising urea, carbamate and free ammonia in aqueous solution to a decomposition unit;
feeding said at least part of the second portion of carbamate in aqueous solution to said decomposition unit,
wherein the treatment of partial decomposition of the reaction mixture and of the second portion of carbamate is carried out advantageously in the same decomposition unit to obtain said first and second flow comprising ammonia and carbon dioxide in vapor phase and a flow comprising urea and residual carbamate in aqueous solution.
In accordance with this embodiment, the implementation of the urea production process is technically very simple, since no relevant additional equipment is required compared to the prior art processes, and it involves low investment costs.
Particularly satisfactory results have been obtained by subjecting at least 50% and preferably at least 65% of said second portion of carbamate in aqueous solution to the treatment of partial decomposition.
According to another aspect of the present invention, the technical problem set forth above is solved by a plant designed to implement the above mentioned urea production process comprising:
a urea synthesis reactor;
a first stripping unit for subjecting a reaction mixture leaving said reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture;
means for condensing at least partially the vapors leaving said first stripping unit and of recycling a first portion of carbamate in aqueous solution to said reactor;
a recovery section of a flow comprising urea and residual carbamate in aqueous solution leaving said first stripping unit for separating the urea produced in the reactor from a second portion of carbamate in aqueous solution;
which plant is characterized in that it comprises:
a second stripping unit for subjecting at least part of said second portion of carbamate in aqueous solution to a treatment of partial decomposition;
means for condensing at least partially the vapors leaving said second stripping unit and of recycling a third portion of carbamate in aqueous solution to said reactor.
In accordance with a still further embodiment of the invention, the plant for urea production comprises:
a urea synthesis reactor;
a stripping unit for subjecting a reaction mixture leaving said first reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture;
means for condensing at least partially the vapors leaving said stripping unit and of recycling a first portion of carbamate in aqueous solution to said first reactor;
a recovery section of a flow comprising urea and residual carbamate in aqueous solution leaving said stripping unit for separating the urea produced in the reactor from a second portion of carbamate in aqueous solution;
which plant is characterized in that it comprises:
means for feeding at least part of said second portion of carbamate in aqueous solution to the stripping unit.
In accordance with the present invention the plants delegated to carry out the urea production process may be provided either new or by modifying pre-existing plants so as to obtain a production capacity expansion and at the same time improved performance from the energy consumption viewpoint.
According to another aspect, the present invention accordingly makes available a method for modernizing a urea production plant of the type comprising:
a urea synthesis reactor;
a first stripping unit for subjecting a reaction mixture leaving said reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said mixture;
means for condensing at least partially the vapors leaving said first stripping unit and of recycling a first portion of carbamate in aqueous solution to said reactor;
a recovery section of a flow comprising urea and residual carbamate in aqueous solution leaving said first stripping unit for separating the urea produced in the reactor from a second portion of carbamate in aqueous solution;
which method is characterized in that it comprises the steps of:
providing a second stripping unit for subjecting at least part of said second portion of carbamate in aqueous solution to a treatment of partial decomposition;
providing means for condensing at least partially the vapors leaving said second stripping unit and of recycling a third portion of carbamate in aqueous solution to said reactor.
In an alternative embodiment, the modernization method of the present invention comprises the steps of
providing a second stripping unit for subjecting at least part of said second portion of carbamate in aqueous solution to a treatment of partial decomposition;
providing means for feeding the vapors leaving said second stripping unit to said means for condensing the vapors leaving said first stripping unit.
According to a further embodiment, the modernization method of the present invention comprises the step of:
providing means for feeding at least part of said second portion of carbamate in aqueous solution to the stripping unit.
Further characteristics and advantages of the present invention are set forth in the detailed description of some preferred embodiments thereof given below by way of non-limiting example with reference to the annexed drawings.